Combined Small-Angle Scattering Analysis of Hierarchically Self-Assembled Soft Matters: Unique "Sponge-Like" Gel Structure as an Example of Dissipative Patterns Formed in Open Non-equilibrium Systems

Takeji Hashimoto

Chinese Journal of Physics

50卷2期（2012 / 04 / 01）

171 - 192

英文

We revisited our previous work [Y. Hirokawa et al. Macromolecules 41, 8210 (2008)] on hierarchically self-assembled ”sponge-like gels” of poly (N-isopropylacrylamide), prepared by a chemical gelation method, by using a ”combined small-angle scattering method” (CSAS) over a wide length scale ranging from ~ nm to ~ μm. The revisitation aimed at gaining a new insight on this chemical gelation process as one of the ”dissipative pattern formation” process in the ”open non-equilibrium systems” in statistical mechanics. The revisitation led us to propose the following scenario for the ”hierarchical dissipative pattern formation” of the sponge-like gels. The pattern formation involves a reaction-induced self-assembly of the molecular systems in three physically different chemical reactions fields: (1) bulk solu- tions, (2) small droplets, and (3) phase-separated domains; The chemical reaction induces a two-step phase separation and a two-step cross-linking reaction, first in the reaction field (2) and then in (3). The more detailed scenario is given as follows. (i) The reaction in the bulk solutions keeps creating ”single-chain globules” in the solution during the whole gelation process at temperatures employed in this work, which in turn creates the reaction fields of (2) and (3) described above. (ii) The reaction in the droplets, formed as a consequence of the ”first-step local phase separation”, causes the ”microgelation” and transforms the droplets into ”microgels” having a tight network, while (iii) the reaction in the phase-separated domains percolated in 3d space, formed as a consequence of the ”second-step phase separation”, causes the ”macrogelation” of the domains with a loose network interconnecting the microgels.